<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    div{
        text-align: center;
        width: 200px;
        height: 200px;
        background-color: red;
        /* 元素水平居中 */
        margin-left: auto;
        margin-right: auto;
    }
    </style>

</head>
<body>
    <div>hello world</div>
</body>
</html>